The present invention relates to a system and method for controlling operation of a fuel cell system having a secondary cell, and more particularly to a system and method for maintaining stable output power of a fuel cell system.
A fuel cell is a power generation system that generates electric energy by a well-balanced electrochemical reaction between hydrogen contained in a hydro-carbonaceous material such as methanol, ethanol and natural gas, and oxygen in the air.
Fuel cells can be sorted according to the type of electrolyte used. Examples include a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte fuel cell and an alkaline fuel cell, etc. These fuel cells are generally operated based on the same principle, but are often different in view of the types of fuels used, operating temperatures, catalyst and electrolytes used, among other differences.
A polymer electrolyte membrane fuel cell (PEMFC) has the advantage of a remarkably high output feature, a low operating temperature, quick start and quick response over other fuel cells. In addition, the PEMFC is widely used as a transportable power source for a portable electronic apparatus or a vehicle as well as a distributed power source such as a stationary power plant for a house and a public structure.
A direct methanol fuel cell (DMFC), which is similar to the PEMFC, supplies directly liquefied methanol fuel to a stack. Unlike the PEMFC, the DMFC does not need to use the reformer to obtain hydrogen from fuel, so it can be small in size.
The typical fuel cell system includes a fuel cell stack for generating power using an electrochemical reaction, a fuel cell stack control circuit, a power supply circuit, where the fuel cell stack control circuit controls the operation of the fuel cell stack. The conventional fuel cell system operates such that it matches its output to the load by controlling a degree of the operation of the fuel cell system depending on the magnitude of the load connected to it. A fuel cell system may also include a secondary cell. In the case of using a secondary cell, the fuel cell is used to charge the secondary cell which can supply the output power of the secondary cell to the load regardless of variation of the load.
However, in the conventional way, the amount or the concentration of the fuel, which is supplied to the fuel cell, may be controlled by the amount of an output, which the fuel cell produces. If the amount or the concentration of the fuel is controlled incorrectly, the operating efficiency and the durability of the fuel cell system are deteriorated. But it is difficult to control mechanically the amount or the concentration of the fluid fuel to meet the magnitude of the load, which is electrically variable at high speed.
In the case using a secondary cell, power is stably supplied to the load from the cell, but the capacity of the secondary cell needs to be large enough. However, enlarging the capacity of the secondary cell is expensive and causes the cost to increase for the whole system.